In recent years, apparatuses which conform to a method such as Moving Picture Experts Group (MPEG) have been widely used for both of information delivery in broadcasting stations or the like and information reception in ordinary homes. MPEG treats image information as a digital value, and, at this time, compresses the image information through orthogonal transform such as discrete cosine transform and motion compensation by using redundancy unique to the image information in order to transmit and accumulate information with high efficiency.
Particularly, MPEG2 (International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 13818-2) is defined as a general use image coding method, and is currently widely used in extensive applications for professional use and consumer use as a standard covering both an interlaced scanning image and a progressive scanning image, and a standard resolution image and a high definition image. By the use of the MPEG2 compression method, it is possible to realize a high compression ratio and good image quality, for example, by assigning a bit rate of 4 Mbps to 8 Mbps to an interlaced scanning image of a standard resolution having 720×480 pixels and assigning a bit rate of 18 Mbps to 22 Mbps to an interlaced scanning image of a high resolution having 1920×1088.
MPEG2 has mainly targeted high image quality coding suitable for broadcasting, but has not handled a coding method at a bit rate lower than that in MPEG1, that is, at a higher compression ratio. With the wide use of portable terminals, the need for such a coding method has been considered to increase, and thus a MPEG4 coding method has been standardized so as to correspond thereto. In relation to an image coding method, a standard thereof was approved as an international standard entitled ISO/IEC 14496-2 on December, 1998.
In addition, in recent years, standardization of a standard called H.26L (International Telecommunication Union Telecommunication Standardization Sector (ITU-T) Q6/16 Video Coding Expert Group (VCEG)) has progressed for the original purpose of image coding for video conference use. H.26L requires a larger calculation amount due to coding and decoding than the coding method of the related art such as MPEG2 or MPEG4, but is known for realizing higher coding efficiency. Further, as part of activities of MPEG4, Joint Model of Enhanced-Compression Video Coding is currently being standardized in order to realize higher coding efficiency by also incorporating functions which are not supported by H.26L on the basis of H.26L.
As for the standardization schedule thereof, the coding method has become an international standard under the name of H.264 and MPEG-4 Part 10 (Advanced Video Coding which will be hereinafter referred to as AVC) on March, 2003.
However, there is a concern that a macroblock size of 16 pixels×16 pixels may not be optimum for a large picture frame such as Ultra High Definition (UHD; 4000 pixels×2000 pixels) which will become a target of a next-generation coding method.
Therefore, currently, for the purpose of improvement in higher coding efficiency than that of AVC, standardization of a coding method called High Efficiency Video Coding (HEVC) is in progress by Joint Collaboration Team-Video Coding (JCTVC) which is a joint standardization organization of ITU-T and ISO/IEC (for example, refer to NPL 1).
In the HEVC coding method, a coding unit (CU) which is the same process unit as the macroblock in the AVC is defined. The CU is not fixed to a size of 16×16 pixels unlike in the macroblock of the AVC, and is designated in image compression information for each sequence.
Meanwhile, a method has been proposed in which, in order to improve coding of a motion vector using median prediction in the AVC, either of “temporal predictor” and “spatio-temporal predictor” is adaptively used as prediction motion vector information in addition to “spatial predictor” which is defined in the AVC and is obtained by the median prediction (for example, refer to NPL 2).
An image information coding apparatus calculates a cost function when prediction motion vector information is used in each block and selects optimal prediction motion vector information. Flag information indicating information regarding which prediction motion vector information is used in each block is transmitted in image compression information.
In addition, as one of the coding methods of motion information, a method (hereinafter, referred to as a merge mode) called motion partition merging has been proposed (for example, refer to NPL 3). In this method, in a case where motion information of a corresponding block is the same as motion information of a neighboring block, only flag information is transmitted, and, during decoding, motion information of the corresponding block is rebuilt by using the motion information of the neighboring block.